It has long been recognised that liquid filled electro-optic displays cells such as electrochromic, electrophoretic or liquid crystal displays must be hermetically sealed so that liquid can neither permeate from the cell nor gas permeate into the cell. The general principles of construction of liquid filled electrochromic displays is described in an article by J Bruinink entitled "Electrochromic Display Devices" (Nonemissive Electro-optic Displays, Ed. A R Kmetz, F K von Willisen, March 1976, p211). This article describes a sandwich type cell construction in which a glass front plate and a glass or steel rear plate are separated by a spacer of glass or a plastics material. Sealing of the spacers and of a filling hole with epoxy resin, glass frit or polymers is mentioned. The purpose is to render the cell hermetic to prevent leakage of oxygen into the cell.
An alternative method of sealing is shown in published French patent application No. 2,505,069 in which a peripheral gasket between two plates of a sandwich type cell is compressed by an external clip acting on the plates. The cell may be additionally externally coated with resin or varnish.
Yet another similar type of sandwich cell construction has been described in published British patent application No. 2,046,935A. This shows a double glass plate cell construction which is sealed at the edges by a double epoxy resin seal, the inner portion of which is cured by reaction with the electrolyte. This publication also lists, among the general requirements of a sealant, that it should have "low permeability against penetration by the electrolyte from the inside and by gases or liquids from the outside".
Japanese published patent application No. 53-93047 shows a similar structure in which the sealant forms an outer wall. A metal film is vapor deposited over the sealant outer wall and also over the edges of the cell base plates in order to prevent moisture in the air from entering the inside of the liquid crystal cell concerned.
In U.S. Pat. No. 4,183,631, a glass topped electrochromic cell with a nickel base has an intermediate side wall made of a polypropylene block. A layer of epoxy resin on the outside of the side wall renders the cell air and water tight.
In allowed U.S. patent application Ser. No. 307,914, U.S. Pat. No. 4,436,378 (also published as European patent application No. 55317-A1), a rather more complex construction of electrochromic cell is described for use in a projection display. To accommodate various features associated with projection, such as an integral side light guide and inwardly sloping outer walls, portions of which are silvered, the entire side wall portion of the cell is moulded from an acrylic plastics material. The transparent cell cover is also acrylic. Although not explicitly described, the cover, side wall and base are bonded together with adhesive. The provision of complex cell wall features, such as the side light guide, is facilitated by the much greater cell depth which can be employed with an electro-chromic cell as opposed to a liquid crystal cell.